NOTE: As their name suggests hints are not exactly Warhammer 40,000: Space Marine cheats, and are usually just simple (but very useful) tips on how you can unlock secret level, features, find hidden areas and such. Hopefully these will make playing Warhammer 40,000: Space Marine even more entertaining.

Achievements
Angel of Death (20): Kill 500 enemies using Ranged weapons.
Armored in Glory (15): Complete 10 armor challenges.
Battle Brother (10): Get a Multiplayer character to Lvl 10.
Blast Radius (5): Multi-kill -- 5 enemies with 1 Frag grenade.
Brute Force...Unleashed (5): Complete a Chapter of the single-player game using only the Vengeance Launcher and Power Axe.
Burn Them All (5): Complete a Chapter of the single-player campaign using only Plasma weapons.
But I Am Finished With You (10): Kill Warboss Grimskull once and for all.
Captain (20): Get a Multiplayer character to Lvl 30.
Chain of Death (10): Multi-kill -- Detonate a chain of 5 consecutive Vengeance Launcher rounds, killing 10 enemies.
Chapter Master (30): Get a Multiplayer character to Lvl 40.
Command Squad (10): Reunite with your Space Marine brothers.
Death from Above (15): Kill 25 enemies with Ground Pound.
Defender (10): Kill 25 opponents who are capturing your Control Point in Seize Ground.
Devastation! (25): Have 10 times more kills than deaths in a single game of Annihilation.
Die, Heretics (20): Kill 50 Chaos Space Marines.
Down to Earth (30): Kill 50 Assault Marines/Raptors in mid-air.
Feel My Wrath (30): Kill 250 enemies using Melee Fury attacks.
Finesse and Fury (5): Complete a Chapter of the single-player game using only Stalker-Pattern Bolter and Chainsword.Firepower (10): Kill 250 enemies using Exotic weapons.
Glorious Slaughter (10): Kill 75 enemies using Executions.
Hammer of the Imperium (10): Use the Invictus to destroy the Orbital Spire.
Here, At the End of All Things (20): Kill Daemon Prince Nemeroth.
Into the Breach (10): Fight your way to the crashed Rok.
Jack of All Trades (25): Play 10 Multiplayer games using each class.
Keeper of the Armory (30): Complete all the weapon challenges.
Lexicanum (10): Collect 10 Servo Skulls.
Librarian of Macragge (75): Collect all Servo Skulls.
Master Crafted (10): Fully customize a Space Marine and a Chaos Space Marine character.
Master of Arms (20): Complete 5 weapon challenges.
Master of Sword and Gun (5): Complete a Chapter of the single-player campaign using only the Bolt Pistol and Chainsword.
Master of the Clean Kill (30): Get 250 Headshots.
Nob Down (20): Win 10 struggles against the Ork Nob.
None Can Stand Before You (125): Complete the entire game on Hard difficulty.
Not So Tough (20): Kill 10 'Ard Boyz.
Precision Killer (5): Multi-kill -- 2 enemies with 1 Stalker-Pattern bolter shot.
Put Them Down (10): Get 100 Headshots.
Shapeshifter (10): Play each class in Multiplayer.
Shock & Awe (20): Kill 150 enemies using the Charge attack.
Silence the Cannon (10): Destroy the Orbital Gun.
Success is Measured in Blood (20): Kill 500 enemies using Melee weapons.
The Bigger They Are... (20): Kill 25 Ork Nobs.
The Emperor Protects (25): Complete Part 1 of the game on Hard difficulty in a single session without dying or restarting.
The Emperor's Marksman (20): Kill 10 enemies in a row in a single Ranged Fury activation.
The Might of the Righteous (5): Kill 100 enemies.
True Son of the Emperor (75): Kill 40,000 enemies in the game (all game modes combined).
Veteran (15): Get a Multiplayer character to Lvl 20.
Visible, Violent Death (30): Kill 2500 enemies.
Warrior of Darkness and Light (15): Play 10 Multiplayer games as Space Marine and 10 Multiplayer games as Chaos Marine.
We Take Our Chances (10): Fire the Psychic Scourge.
You Must Carry It (5): Retrieve the Power Source.